1. Field of the Invention
The present invention relates to a color imaging element and an imaging device, in particular, to a color imaging element capable of reducing occurrence of color moire and realizing high resolution, and an imaging device using such a color imaging element.
2. Description of the Related Art
In a single-plate color imaging element, since a monochromatic color filter is set on each pixel, each pixel has only monochrome color information. Therefore, since an output image of the single-plate color imaging element is a RAW image (mosaic image), a multi-channel image is acquired by processing (demosaicing process) that interpolates a missing color pixel with a surrounding pixel. In this case, there is a problem in reproduction characteristic of a high-frequency image signal, and aliasing easily occurs in a taken image of the color imaging element as compared with a black-and-white imaging element. Therefore, it is an important problem to expand a reproduction band and achieve high resolution while reducing the occurrence of color moire (false color).
In a primary color Bayer array that is a color array of a color filter that is used in the single-plate color imaging element most widely, green (G) pixels are disposed in a checkered pattern and red (R) and blue (B) are disposed in a line-sequential manner. Therefore, there is a problem in reproduction precision when generating a high frequency signal in which G signals are in the oblique direction and R and B signals are in the horizontal and vertical directions.
In a case where a black-and-white stripe pattern (high-frequency image) as illustrated in portion (A) of FIG. 22 enters a color imaging element having a color filter of the Bayer array illustrated in portion (B) of FIG. 22, when this is distributed according to the Bayer color array and comparison is performed in each color, as illustrated in portions (C) to (E) of FIG. 22, R becomes a mosaic color image of light and flat, B becomes a mosaic color image of dark and flat, and G becomes a mosaic color image of light and shade. That is, it is essentially a black-and-white image and concentration difference (level difference) is not caused among RGB, but it enters a state where the image is colored depending on the color array and an input frequency.
Similarly, in a case where an oblique black-and-white high-frequency image as illustrated in portion (A) of FIG. 23 enters an imaging element having a color filter of the Bayer array illustrated in portion (B) of FIG. 23, when this is distributed according to the Bayer color array and comparison is performed in each color, as illustrated in portions (C) to (E) of FIG. 23, R and B become a color image of light and flat and G becomes a color image of dark and flat. If a value of black is assumed to be 0 and a value of white is assumed to be 255, only G becomes 255 and therefore the oblique black-and-white high-frequency image becomes green. Thus, in the Bayer array, it is not possible to correctly reproduce an oblique high-frequency image.
In general, it is avoided by disposing an optical low-pass filter including a birefringent material such as crystal in front of the color imaging element and optically decreasing the high frequency in the imaging device using single-plate color imaging elements. However, it is possible to mitigate coloring by the aliasing of high-frequency signals in this method, but there is a problem that resolution degrades due to the negative effect.
To solve such a problem, there is suggested a color imaging element where a color filter array of a color imaging element is assumed to be a three-color random array satisfying array restriction conditions in which an arbitrary target pixel is adjacent to three colors including a color of the target pixel on any of four sides of the target pixel (Japanese Patent Application Laid-Open No. 2000-308080: PTL 1).
Moreover, there is suggested an image sensor of a color filter array (color imaging element) where the image sensor includes a plurality of filters with different spectral sensitivities, and a first filter and a second filter among them are alternately disposed in a first period in one diagonal direction of a pixel grid of the image sensor and are alternately disposed in a second period in the other diagonal direction (Japanese Patent Application Laid-Open No. 2005-136766: PTL 2).
In addition, there is suggested a color array where, in a color solid-state imaging element of three primary colors of RGB (color imaging element), a set of three pixels in which R, G and B are horizontally disposed are disposed in a zigzag manner in a vertical direction to make appearance probabilities of RGB respectively equal and to cause an arbitrary line (horizontal, vertical and oblique lines) on an imaging plane to pass through all colors (Japanese Patent Application Laid-Open No. 11-285012: PTL 3).
Furthermore, there is suggested a color imaging element where R and B among three primary colors of RGB are disposed every three pixels in horizontal and vertical directions and G is disposed between R and B (Japanese Patent Application Laid-Open No. 8-023543: PTL 4).